Glass composition, glass composition for chemical strengthening, strengthened glass article, and cover glass for display

ABSTRACT

Provided is a glass composition containing, in mol %: 56 to 66% SiO 2 ; 6 to 12% Al 2 O 3 ; 5 to 14% MgO; 0 to 1% CaO; 17 to 24% Na 2 O; and 0 to 3% K 2 O. The total content of Li 2 O, Na 2 O, and K 2 O is in a range of 18.5 to 24%. This glass composition is suitable for production by a float process and for introduction of a compressive stress layer with a high crack initiation load, a high surface compressive stress, and a large thickness by chemical strengthening.

TECHNICAL FIELD

The present invention relates to a glass composition suitable forchemical strengthening, more specifically to a glass composition havingproperties suitable for use as a cover glass of a display. The presentinvention also relates to a glass composition for chemicalstrengthening, a chemically-strengthened strengthened glass article, anda cover glass for a display.

BACKGROUND ART

In recent years, electronic devices with liquid crystal displays,organic EL displays, etc. and electronic devices with touch paneldisplays have been widespread. Since glass materials have high surfacehardness, they are widely used as materials of cover glasses of displaysof these electronic devices. Cover glasses of displays are sometimeschemically strengthened to improve their mechanical strength.

Chemical strengthening is a technique of replacing alkali metal ionscontained in the glass surface by monovalent cations having a largerionic radius so as to form a compressive stress layer on the glasssurface. Chemical strengthening is often performed by replacing lithiumions (Li⁺) by sodium ions (Na⁺) or by replacing sodium ions by potassiumions (K⁺).

A glass composition suitable for chemical strengthening disclosed inPatent Literature 1 contains 64 to 68 mol % SiO₂, 12 to 16 mol % Na₂O,and 8 to 12 mol % Al₂O₃. In this glass composition, the content of Na₂Ois higher than that of Al₂O₃ by 2 to 6 mol %, and the total content ofalkaline earth metal oxides (MgO+CaO+SrO) is adjusted to 5 to 8 mol %(claim 1). In addition, the glass composition described in PatentLiterature 1 has a melting temperature of less than 1650° C. and aliquidus viscosity of at least 13 kPa·s to be adapted to a down-drawprocess. In the glass compositions described as examples in PatentLiterature 1, the contents of Al₂O₃ and Na₂O are 8.9 mol % or more and14.38 mol % or less, respectively.

A strengthened glass substrate suitable for use in a touch panel displaydisclosed in Patent Literature 2 contains, in mass %, 45 to 75% SiO₂, 1to 30% Al₂O₃, 0 to 20% Na₂O, and 0 to 20% K₂O (claim 3). Furthermore, inthe examples, the temperatures at which the glass substrates have aviscosity of 10⁴ dPa·s are 1122° C. to 1414° C. These glass substratesare suitable for production by a down-draw process.

A working temperature and a melting temperature are known measures ofthe high-temperature viscosity of glass. In a float process, the workingtemperature is a temperature at which glass has a viscosity of 10⁴dPa·s, and hereinafter may be referred to as T₄. The melting temperatureis a temperature at which glass has a viscosity of 10² dPa·s, andhereinafter may be referred to as T₂.

CITATION LIST Patent Literature

Patent Literature 1: JP 2010-527892 T

Patent Literature 2: JP 2010-116276 A

SUMMARY OF INVENTION Technical Problem

Glass compositions having a low T₂ and a low T₄, in particular, glasscompositions having a low T₄ are suitable for production by the floatprocess. On the other hand, glass compositions for chemicalstrengthening are expected to be susceptible to ion exchange byreplacement of cations so as to introduce a compressive stress layerwith a high crack initiation load, a high surface compressive stress,and a large thickness.

In view of the above circumstances, it is an object of the presentinvention to provide a glass composition suitable for production by afloat process and for introduction of a compressive stress layer with ahigh crack initiation load, a high surface compressive stress, and alarge thickness by chemical strengthening.

Solution to Problem

In order to achieve the above object, the present invention provides aglass composition containing, in mol %: 56 to 66% SiO₂; 6 to 12% Al₂O₃;5 to 14% MgO; 0 to 1% CaO; 17 to 24% Na₂O; and 0 to 3% K₂O. The totalcontent of Li₂O, Na₂O, and K₂O is in a range of 18.5 to 24%.

In another aspect, the present invention provides a strengthened glassarticle including a compressive stress layer formed as a surface of thestrengthened glass article by bringing a glass article containing theglass composition of the present invention into contact with a moltensalt containing monovalent cations having an ionic radius larger thanthat of sodium ions so as to allow ion exchange to take place betweensodium ions contained in the glass composition and the monovalentcations.

The present invention further provides a cover glass for a display, thecover glass including the strengthened glass article of the presentinvention.

Advantageous Effects of Invention

The glass composition according to the present invention has arelatively low T₄ and thus is suitable for production by the floatprocess. Furthermore, the glass composition of the present invention issuitable for obtaining a strengthened glass article having a compressivestress layer with a high crack initiation load, a large thickness, and ahigh surface compressive stress.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the percentages of the components of glass compositions areall expressed in mol %, unless otherwise specified. In this description,the phrase “consisting essentially of components” means that the totalcontent of the components referred to is 99.5 mol % or more, preferably99.9 mol % or more, and more preferably 99.95 mol % or more. The phrase“being substantially free of a component” means that the content of thecomponent is 0.1 mol % or less, and preferably 0.05 mol % or less.

The glasses disclosed in Patent Literature 1 and Patent Literature 2have high high-temperature viscosities and high T₄ values. A high T₄value is disadvantageous in producing a cover glass of a display by thefloat process, and is also disadvantageous in forming glass into a thinsheet as a cover glass of a display.

The present invention is intended to reduce the T₄ by a thorough studyof, in particular, the contents of Al₂O₃, Na₂O, alkaline earth oxides,and alkali metal oxides in view of the effects of each of these oxideson the properties and by an overall adjustment of the contents of theother components, and thereby to provide a glass composition suitablefor production by the float process, in particular, a glass compositionadvantageous in forming glass into a thinner sheet (for example, with athickness of 1 mm or less) as a cover glass for a display and resistantto scratching and cracking.

The following points are not essential in the present invention.However, in the present invention, attention can be given to thefollowing points in some cases.

The present invention is intended to provide a glass composition havinga relatively low T₂ so as to be adapted to a glass melting furnace usedin conventional production facilities for the float process. The presentinvention is also intended to provide a glass composition in which avalue obtained by subtracting the liquidus temperature TL from the T₄ isa not too large negative value or a positive value (for example, −10° C.or more, preferably 0° C. or more, and more preferably 10° C. or more)so as to be adapted to glass formation by the float process.

Hereinafter, the components of the glass composition of the presentinvention are described respectively.

(SiO₂)

SiO₂ is the main component of a glass composition. An excessively lowcontent of SiO₂ reduces the chemical durability such as water resistanceand heat resistance of the glass. On the other hand, an excessively highcontent of SiO₂ increases the viscosity of the glass composition at hightemperatures and thus makes it difficult to melt and form the glasscomposition. Therefore, the appropriate content of SiO₂ is in a range of56 to 66%. The content of SiO₂ is preferably 57 to 64%, and morepreferably 57 to 62%.

(Al₂O₃)

Al₂O₃ improves the chemical durability such as water resistance of aglass composition and further facilitates migration of alkali metal ionsin the glass. Al₂O₃ is also a component that contributes to maintainingthe strength obtained by chemical strengthening. On the other hand, anexcessively high content of Al₂O₃ increases the viscosity of the glassmelt, and thus increases the T₂ and T₄ and reduces the clarity of theglass melt, which makes it difficult to produce a high quality glasssheet.

Therefore, the appropriate content of Al₂O₃ is in a range of 6 to 12%.The content of Al₂O₃ is preferably 11% or less, and more preferably 10%or less. The content of Al₂O₃ is preferably 7% or more, and morepreferably 8% or more.

As for the contents of SiO₂ and Al₂O₃, the glass compositions close toand including the glass composition of the present invention have thefollowing features.

In order to produce a glass sheet by the float process and to performchemical strengthening treatment at a relatively low temperature and ina short time, a glass composition needs to contain a certain amount ofNa₂O. Therefore, the glass composition of the present invention contains17% or more Na₂O.

On the other hand, it was found that the glass compositions can beclassified into two groups according to the following parameters ofglass articles obtained by chemical strengthening: the surfacecompressive stress and the depth of the compressive stress layer; andthe crack initiation load defined as an indentation load at which cracksemanating from an indentation formed by a Vickers indenter occur with aprobability of 50%.

The first group of glass compositions include those that can provide asurface compressive stress of 750 MPa or more after any of chemicalstrengthening treatments that were performed thereon. In the first groupof glass compositions, there is no correlation between the surfacecompressive stress and the depth of the compressive stress layer and thecrack initiation load of the chemically-strengthened glass article,which reveals that the crack initiation load of the glass article isdetermined solely by the glass composition thereof.

The second group of glass compositions include those that can provide asurface compressive stress of less than 750 MPa after any of chemicalstrengthening treatments that were performed thereon. In the secondgroup of glass compositions, there is a strong positive correlationbetween the crack initiation load of the chemically-strengthened glassarticle and the surface compressive stress, and thus the crackinitiation load decreases rapidly as the surface compressive stressdecreases. In addition, in all the glass compositions belonging to thesecond group, the crack initiation load had very low values.

In the present invention, based on the above-described findings, a glasscomposition belonging to the above-mentioned first group and having aSiO₂ content of 66% or less and an Al₂O₃ content of 6% or more wasselected. With the use of the glass composition of the present inventionin which the contents of SiO₂ and Al₂O₃ satisfy the above conditions andthe contents of the other components are appropriately adjusted, it ispossible to obtain a strengthened glass article having a compressivestress layer with a surface compressive stress of 900 MPa or more, acrack initiation load of 3.9 kgf or more, and a thickness of 25 μm ormore, and even a strengthened glass article having a compressive stresslayer with a surface compressive stress of 1000 MPa or more and a depthof 30 μm or more.

However, there is a positive correlation between the difference obtainedby subtracting the content of Al₂O₃ from the content of SiO₂ (Si₂—Al₂O₃)and acid resistance of the glass composition. When a glass articlehaving a low acid resistant glass composition is immersed in an acidsolution such as an aqueous hydrofluoric acid solution, the surface ofthe glass article is damaged, regardless of whether it is subjected tochemical strengthening treatment or not. In view of this, in the glasscomposition of the present invention, the content of SiO₂ is 56% or moreand the content of Al₂O₃ is 12% or less.

(Na₂O)

Na₂O is a component that increases the surface compressive stress andthus increases the depth of the surface compressive stress layer whensodium ions are replaced by potassium ions. However, if the content ofNaO is higher than the appropriate content thereof, the surfacecompressive stress relaxed in the chemical strengthening treatment isgreater than the stress produced by ion exchange in the chemicalstrengthening treatment, and as a result, the surface compressive stressis likely to decrease.

Na₂O is a component that increases the meltability and reduces the T₄and T₂. On the other hand, an excessively high content of Na₂Osignificantly reduces the water resistance of glass.

Therefore, in the glass composition of the present invention, theappropriate content of Na₂O is in a range of 17 to 24%. The content ofNa₂O is preferably 18.5% or more, and more preferably 19% or more. Thecontent of Na₂O is preferably 22% or less, and more preferably 21% orless. However, in order to ensure reduction of the T₄, etc., the contentof Na₂O may be 22% or more depending on the contents of the othercomponents.

(MgO)

MgO is most effective in increasing the meltability of glass in alkalineearth oxides (RO components). In order to obtain this effectsufficiently, in the glass composition of the present invention, thecontent of MgO is 5% or more. On the other hand, an excessively highcontent of MgO beyond its appropriate content rapidly reduces thestrengthening effects obtained by chemical strengthening, in particular,the depth of the surface compressive stress layer, and also reduces thecrack initiation load. Among the RO components, MgO is least likely tohave these negative effects, but in the glass composition of the presentinvention, the content of MgO is 14% or less. In addition, a highcontent of MgO increases the liquidus temperature TL of the glasscomposition.

Therefore, in the glass composition of the present invention, thecontent of MgO is in a range of 5 to 14%. The content of MgO ispreferably 7% or more, and more preferably 8% or more. The content ofMgO is preferably 12% or less, and more preferably 11% or less.

(CaO)

CaO has the effect of reducing the viscosity at high temperatures.However, an excessively high content of CaO inhibits migration of sodiumions in a glass composition and makes the glass composition moresusceptible to devitrification. However, it is preferable to add CaObecause a small amount of CaO is effective in lowering the liquidustemperature TL.

Therefore, the appropriate content of CaO is in a range of 0 to 1%. Thecontent of CaO is preferably 0.7% or less, and more preferably 0.5% orless. The content of CaO may be 0.3% or more.

(SrO and BaO)

SrO and BaO are more effective than CaO in significantly reducing theviscosity of a glass composition and reducing the liquidus temperatureTL of the glass composition, if their contents are low. Even if thecontents of SrO and BaO are very low, they significantly inhibitmigration of sodium ions in the glass composition and has a significantnegative effect on both the surface compressive stress and the depth ofthe compressive stress layer.

Therefore, it is preferable that the glass composition of the presentinvention be substantially free of SrO and BaO.

(K₂O)

Like Na₂O, K₂O is a component that increases the meltability of glass. Alow content of K₂O increases the ion exchange rate in chemicalstrengthening, increases the depth of the compressive stress layer, andat the same time lowers the devitrification temperature TL of a glasscomposition. Therefore, it is preferable that the glass composition havea low content of K₂O.

On the other hand, K₂O impairs the clarity of glass melt compared toNa₂O. An excessively high content of K₂O is more likely to reduce thecrack initiation load after chemical strengthening. In addition, as thecontent of K₂O increases, a molten salt used in chemical strengtheningdecomposes and the chemical strengthening effect is more likely todecrease accordingly.

Therefore, the appropriate content of K₂O is in a range of 0 to 3%. Thecontent of 1(20 is preferably 1.5% or less, and more preferably 1% orless. The content of K₂O may be 0.2% or more, and even 0.5% or more.

(Li₂O)

Li₂O significantly reduces the depth of a compressive stress layer evenif the content of Li₂O is very low. When a glass article containing Li₂Ois subjected to chemical strengthening treatment in a molten salt ofpotassium nitrate alone, the molten salt decomposes at a much higherrate than in the case of a glass article free of Li₂O. Specifically, inthe case where the chemical strengthening treatment is performedrepeatedly using the same molten salt, the quality of the propertiesobtained by chemical strengthening degrade more rapidly, that is, thequality of the properties obtained degrade with fewer repetitions of thetreatment. Therefore, it is preferable that the glass composition of thepresent invention be substantially free of Li₂O.

(R₂O)

R₂O refers to Li₂O, Na₂O, and K₂O. If the content of R₂O is too low, theamount of the components that reduce the viscosity of a glasscomposition is too small, which makes it difficult to melt the glasscomposition. On the other hand, when the glass composition of thepresent invention is subjected to chemical strengthening, ions derivedfrom the molten salt diffuse into the glass to produce a compressivestress, but the compressive stress is likely to relax and decrease dueto the relaxation of the glass structure, depending on the balancebetween the content of R₂O and the contents of Al₂O₃ and MgO. In orderto minimize this negative effect, the upper limit is put on the contentof R₂O in the glass composition of the present invention.

Therefore, the appropriate content of R₂O (appropriate total content ofLi₂O, Na₂O, and K₂O) is in a range of 18.5 to 24%. The content of R₂O ispreferably 19% or more, and preferably 22% or less. However, in order toensure reduction of the T₄, etc., the content of R₂O may be 22% or moredepending on the contents of the other components.

(B₂O₃)

B₂O₃ is a component that reduces the viscosity of a glass compositionand improves the meltability thereof. However, an excessively highcontent of B₂O₃ makes the glass composition more susceptible to phaseseparation and reduces the water resistance of the glass composition. Inaddition, compounds formed from B₂O₃ and alkali metal oxides mayvaporize and damage the refractory material of a glass melting chamber.Furthermore, the addition of B₂O₃ causes a decrease in the depth of thecompressive stress layer formed by chemical strengthening. Therefore,the appropriate content of B₂O₃ is 3% or less. In the present invention,it is more preferable that the glass composition be substantially freeof B₂O₃.

(Fe₂O₃)

Fe is normally present in the form of Fe²⁺ or Fe³⁺ in glass. Fe³⁺ is acomponent that improves the ultraviolet ray absorbing properties ofglass, and Fe²⁺ is a component that improves the heat ray absorbingproperties of glass. However, when the glass composition is used for acover glass of a display, it is preferable to minimize the content of Feto prevent the glass composition from being conspicuously colored. Femay be inevitably mixed in the glass composition due to an industrialraw material, but it is recommended that the content of total iron oxidebe 0.1% or less, and preferably 0.02% or less, as calculated in terms ofFe₂O₃ content. In the present invention, the glass composition may besubstantially free of iron oxide.

(TiO₂)

TiO₂ is a component that reduces the viscosity of a glass compositionand increases the surface compressive stress produced by chemicallystrengthening. However, a high content of TiO₂ colors the glasscomposition yellow, which is not desired. Therefore, the appropriatecontent of TiO₂ is 0 to 1%. There may be a case where TiO₂ is inevitablymixed in the glass composition due to an industrial raw material and theglass composition contains 0.05% TiO₂, but this low content of TiO₂ doesnot cause undesirable coloring.

(ZrO₂)

ZrO₂ is a component that increases the water resistance of glass andfurther increases the surface compressive stress produced by chemicallystrengthening. However, a high content of ZrO₂ may cause a rapidincrease in the liquidus temperature TL. Therefore, the appropriatecontent of ZrO₂ is 0 to 1%. In the present invention, the glasscomposition may be substantially free of ZrO₂.

(SO₃)

In the float process, a sulfate such as sodium sulfate (Na₂SO₄) iswidely used as a refining agent. A sulfate is decomposed in molten glassto produce a gas component, which promotes degassing of the glass melt,but a portion of the gas component is dissolved in the form of SO₃ andremains in the glass composition. In the glass composition of thepresent invention, the content of SO₃ is preferably 0.1 to 0.3%.

(SnO₂)

It is known that, in formation of a glass sheet by the float process,molten tin in a tin bath diffuses into the glass in contact with the tinbath so as to be present in the form of SnO₂. SnO₂ also contributes todegassing when it is mixed as one of the glass raw materials. In theglass composition of the present invention, the content of SnO₂ ispreferably 0 to 0.4%.

(Other Components)

Preferably, the glass composition of the present invention consistsessentially of the components (from Al₂O₃ to SnO₂) mentioned above. Theglass composition of the present invention may contain components otherthan the above-mentioned components. In this case, the content of eachof the other components is preferably less than 0.1%.

Examples of the other components that the glass composition may containinclude As₂O₅, Sb₂O₅, CeO₂, Cl, and F in addition to the above-mentionedSO₃ and SnO₂. These components are added to degas the molten glass.However, it is preferable not to add As₂O₅, Sb₂O₅, Cl, and F becausethey have serious adverse effects on the environment. Other examples ofthe components that the glass composition may contain include ZnO, P₂O₅,GeO₂, Ga₂O₃, Y₂O₃, and La₂O₃. The glass composition may containcomponents other than the above-mentioned components derived fromindustrially available raw materials, unless the content of each ofthese components exceeds 0.1%. Since these components are optionallyadded if necessary or are inevitably mixed, the glass composition of thepresent invention may be substantially free of these components.

Hereinafter, the properties of the glass composition of the presentinvention are described.

(Glass Transition Temperature: Tg)

According to the present invention, it is possible to provide a glasscomposition having a glass transition temperature (Tg) of 610° C. orless, further 590° C. or less, or even 570° C. or less in some cases,and thus it is easier to slowly cool molten glass to produce the glasscomposition. The lower limit of the glass transition temperature is notparticularly limited, and it may be 530° C. or more, preferably 550° C.or more to prevent relaxation of the compressive stress produced by ionexchange.

(Working Temperature: T₄)

In the float process, the viscosity of molten glass is adjusted to about10⁴ dPa·s (10⁴ P) when the molten glass in a melting furnace is pouredinto a float bath. In the production by the float process, it ispreferable that the temperature (working temperature: T₄) at which themolten glass has a viscosity of 10⁴ dPa·s be lower. For example, inorder to form the glass into a thin sheet for use as a cover glass of adisplay, the working temperature T₄ of the molten glass is preferably1100° C. or less. According to the present invention, it is possible toprovide a glass composition having a T₄ of 1090° C. or less, further1075° C. or less, or even 1060° C. or less in some cases and thussuitable for production by the float process. The lower limit of the T₄is not particularly limited, and it is 1000° C., for example.

(Melting Temperature: T₂)

When the temperature (melting temperature: T₂) at which the molten glasshas a viscosity of 10² dPa·s is low, the amount of energy required tomelt the glass raw materials can be reduced, and the glass raw materialscan be more easily dissolved to promote degassing and refining of theglass melt. According to the present invention, it is possible to reducethe T₂ to 1550° C. or less, and even 1530° C. or less.

(Difference Between Working Temperature and Liquidus Temperature: T₄-TL)

In the float process, it is preferable that molten glass does notdevitrify when the temperature of the molten glass is T₄. In otherwords, it is preferable that the difference between the workingtemperature (T₄) and the liquidus temperature (TL) be large. Accordingto the present invention, it is possible to provide a glass compositionin which a difference obtained by subtracting the liquidus temperaturefrom the working temperature is as large as −10° C. or more, and even 0°C. or more. In addition, according to the present invention, it ispossible to reduce the TL to 1050° C. or less, and even 1000° C. or lessso as to contribute to increasing the difference T₄ TL.

(Density (Specific Gravity): d)

It is desirable that a cover glass of a display for an electronic devicehave a low density to reduce the weight of the electronic device.According to the present invention, it is possible to reduce the densityof the glass composition to 2.53 g·cm⁻³ or less, further 2.51 g·cm⁻³ orless, and even 2.50 g·cm⁻³ or less in some cases.

In the float process or the like, when production of glass is changedfrom one type of glass to another type of glass, if there is a largedifference in the density between these two types of glass, a portion ofone type of glass having a higher density melts and remains at thebottom of a melting furnace, which may affect the changeover toproduction of another type of glass. The density of soda lime glass,which is currently mass-produced by the float process, is about 2.50g·cm⁻³. Therefore, for the mass production by the float process, it ispreferable that the glass composition has a density close to the valuementioned above. Specifically, the density of the glass composition ispreferably 2.45 to 2.55 g·cm⁻³, and particularly preferably 2.47 to 2.53g·cm⁻³.

(Elastic Modulus: E)

When a glass substrate is subjected to chemical strengthening by ionexchange, it may be bent. It is preferable that the glass compositionhave a high elastic modulus to reduce this bending. According to thepresent invention, it is possible to increase the elastic modules(Young's modulus: E) of the glass composition to 70 GPa or more, andeven to 72 GPa or more.

(Thermal Expansion Coefficient: α)

According to the present invention, it is possible to provide a glasscomposition having a linear thermal expansion coefficient in a range of95×10⁻⁷/° C. to 112×10⁻⁷/° C. in a temperature range of 50 to 350° C.The glass composition having a linear thermal expansion coefficient inthis range has the advantage of being less susceptible to bending ordistortion when it is attached to a material having a higher linearthermal expansion coefficient than the linear thermal expansioncoefficients (70×10⁻⁷/° C. to 100×10⁻⁷/° C.) of common glass members.

According to a preferred embodiment of the present invention, it ispossible to provide a glass composition having a linear thermalexpansion coefficient in a range of 100×10⁻⁷/° C. or more in atemperature range of 50 to 350° C.

(Crack Initiation Load: Rc)

Cover glasses of displays are expected to be resistant to scratching andcracking. For the glass composition of the present invention, the crackinitiation load determined by a test described later was used as ameasure of the resistance to scratching and cracking of the glasssurface. The crack initiation load of the strengthened glass article ofthe present invention is 3.9 kgf (kilogram force) or more, and can beincreased to 4 kgf or more, to 5 kgf or more in some cases, and even to5.2 kgf or more.

The chemical strengthening of the glass composition is described below.

(Conditions of Chemical Strengthening and Compressive Stress Layer)

Chemical strengthening of the glass composition of the present inventioncan be performed by bringing the glass composition containing sodiuminto contact with a molten salt containing monovalent cations,preferably potassium ions, having an ionic radius larger than that ofsodium ions, so as to allow ion exchange to take place between sodiumions in the glass composition and the monovalent cations in the form ofreplacement of the sodium ions by the monovalent cations. Thus, acompressive stress layer having a surface compressive stress is formed.

A typical example of the molten salt is potassium nitrate. A molten saltmixture of potassium nitrate and sodium nitrate also can be used, but itis preferable to use potassium nitrate alone because it is difficult tocontrol the concentration of a molten salt mixture.

The surface compressive stress and the depth of the compressive stresslayer of a strengthened glass article can be controlled not only by theglass composition of the article but also by the temperature of themolten salt and the treatment time in the ion exchange treatment.

It is possible to obtain a strengthened glass article having acompressive stress layer with a very high crack initiation load, amoderately large thickness, and a moderately high surface compressivestress by bringing the glass composition of the present invention intocontact with a molten salt of potassium nitrate.

Specifically, it is possible to obtain a strengthened glass articlehaving a compressive stress layer with a surface compressive stress of900 MPa or more and a depth of 25 μm or more, and in addition, with acrack initiation load of 3.9 kgf or more, further 5 kgf or more, andeven 6 kgf in some cases.

Since this strengthened glass article has a very high crack initiationload, its surface is resistant to cracking and scratching and has astrength suitable for use as a cover glass of a display.

It is also possible to obtain a strengthened glass article having acompressive stress layer with a very high surface compressive stress anda very large thickness by bringing the glass composition of the presentinvention into contact with a molten salt of potassium nitrate.Specifically, it is possible to obtain a strengthened glass articlehaving a compressive stress layer with a surface compressive stress of1000 MPa or more, even 1200 MPa or more or 1400 MPa or more in somecases, and a thickness of 30 μm or more, even 40 μm or more or 50 μm ormore in some cases.

Since this strengthened glass article has a very high surfacecompressive stress, its surface is resistant to scratching. In addition,since the strengthened glass article has a compressive stress layer witha very large thickness, even if the surface has a scratch, the scratchis less likely to develop into the glass article due to the presence ofthe compressive stress layer and thus is less likely to damage thestrengthened glass article. This strengthened glass article has astrength suitable for use as a cover glass of a display.

According to the present invention, it is possible to provide a glasscomposition having a relatively low T₄, suitable for production by thefloat process, and advantageous in forming glass into a thin glass sheetfor use as a cover glass of a display.

The strengthened glass article obtained by chemically strengthening theglass composition of the present invention is suitable for use as acover glass of a liquid crystal display, an organic EL display, atouch-panel display, or the like for an electronic device. It should benoted that the glass composition of the present invention does notnecessarily have to be subjected to chemical strengthening treatment,and the untreated glass composition also can be used as a substrate foran electronic device or the like.

EXAMPLES

Hereinafter, the present invention will be described in more detail byway of examples and comparative examples.

(Preparation of Glass Compositions)

As commonly available glass raw materials such as silica, titaniumoxide, alumina, sodium carbonate, potassium carbonate, basic magnesiumcarbonate, calcium carbonate, strontium carbonate, and barium carbonatewere used to prepare glass formulations (batches) having the glasscompositions shown in Tables 1 to 6. In some examples, sodium sulfatewas used instead of sodium carbonate. In Comparative Examples 8 and 9,zirconium oxide, boron oxide, and tin (IV) oxide were further added tothe glass formulations. The batches thus prepared were each put into aplatinum crucible and heated in an electric furnace at 1550° C. for 4hours. Thus, a molten glass was obtained. Next, the molten glass waspoured on an iron plate for cooling to obtain a glass plate. Next, thisglass plate was again placed in the electric furnace at 600° C. for 2hours. Then, the furnace was turned off to slowly cool the glass plateto room temperature. Thus, a glass sample was obtained.

For each glass sample, the glass transition temperature Tg, the glasssoftening point Ts, the working temperature T₄, the melting temperatureT₂, the liquidus temperature TL, the thermal expansion coefficient α,the density d, and the Young's modulus E were measured.

The glass transition temperature Tg and the thermal expansioncoefficient a were measured using a differential thermal analyzer(Thermoflex TMA 8140, manufactured by Rigaku Corporation). The workingtemperature T₄ and the melting temperature T₂ were measured by aplatinum ball pulling-up method. The density d was measured by anArchimedes method. The Young's modulus E was measured according to JIS(Japanese Industrial Standards) R 1602.

The liquidus temperature TL was measured in the following manner. Theglass sample was pulverized and sieved. Glass particles that passedthrough a 2380-μm mesh sieve but retained on a 1000-μm mesh sieve wereobtained. These glass particles were immersed in ethanol and subjectedto ultrasonic cleaning, followed by drying in a thermostat. 25 g of theglass particles were placed in a platinum boat having a width of 12 mm,a length of 200 mm and a depth of 10 mm so as to obtain a measurementsample with a constant thickness. This platinum boat was placed in anelectric furnace (a temperature gradient furnace) with a temperaturegradient from about 850 to 1200° C. for 24 hours. Then, the measurementsample was observed using an optical microscope with a magnification of100, and the highest temperature in a region where devitrification wasobserved was determined to be the liquidus temperature of the sample. Inall Examples and Comparative Examples, glass particles in themeasurement samples were fused together to form rods in the temperaturegradient furnace.

(Production of Strengthened Glass)

The glass sample thus obtained was cut into pieces of 25 mm×35 mm. Bothsurfaces of each piece were polished with alumina abrasive grains andfurther mirror-polished with cerium oxide abrasive grains. Thus, two ormore 5 mm-thick glass blocks having surfaces with a surface roughness Raof 2 nm or less (a surface roughness Ra according to JIS B 0601-1994)were obtained for each composition. These glass blocks were immersed ina molten salt of potassium nitrate at predetermined temperatures rangingfrom 380° C. to 420° C., respectively, for 4 to 8 hours so as to allowion exchange (I/E) to take place and thus to chemically strengthen theglass blocks. After the chemical strengthening treatment, the glassblocks were washed with hot water at 80° C. Thus, strengthened glassblocks were obtained.

In order to reduce the thermal shock applied to the glass blocks, theywere preheated before being immersed in the molten salt and were slowlycooled after being immersed in the molten salt (that is, after beingremoved from the molten salt). Preheating was performed by placing theglass blocks in a space above the level of the molten salt in acontainer for 10 minutes. Slow cooling was also performed in the samemanner as preheating. This slow cooling also has the effect of returningthe molten salt remaining on the removed glass blocks as much aspossible to the molten salt container.

For the strengthened glass blocks thus obtained, the surface compressivestress CS and the compression depth (the depth of the compressive stresslayer) DOL were measured using a surface stress meter “FSM-6000”manufactured by Orihara Industrial Co., Ltd. Tables 1 to 6 collectivelyshow the results.

(Evaluation of Crack Initiation Load Rc)

For some of the strengthened glass blocks obtained as described above,the crack initiation load was evaluated. The crack initiation load wascalculated in the following manner using a Vickers hardness testermanufactured by Akashi Corporation. First, a Vickers indenter waspressed against the surface of the glass sample and applied a load of 1kgf thereto for 15 seconds. 5 minutes after removal of the load, thenumber of cracks emanating from the corners of a square indentation onthe surface of the glass sample was counted. This counting was repeated10 times, and the total number of cracks was divided by 40, which is thetotal number of the corners of the indentation for 10 times, so as tocalculate the crack occurrence probability P. The level of the loadapplied was increased to 2 kgf, 5 kgf, 10 kgf, and 20 kgf step by step,and the crack occurrence probability P was calculated at each of theloads in the same manner as described above. Thus, the two adjacentloads WH and WL, between which the probability of 50% (P=50%) occurred,and the crack occurrence probabilities PH and PL at these two adjacentloads (PL<50%<PH) were obtained. The load at which a straight lineconnecting two points (WH, PH) and (WL, PL) passed through the point ofP=50% was obtained and defined as the crack initiation load Rc. Tables 1to 6 collectively show the results.

In most Examples, the glass transition temperatures Tg were 610° C. orless and the working temperatures T₄ were 1100° C. or less. In someExamples, the melting temperatures T₂ measured were 1550° C. or less. Inall Examples, the differences T₄ TL each obtained by subtracting theliquidus temperature TL from the working temperature T₄ were −1° C. ormore. In Examples, the densities d were 2.48 to 2.52 g·cm⁻³.

In all Examples, strengthened glass articles each having a compressivestress layer with a very high surface compressive stress (1100 MPa ormore) and a moderately large thickness (25 μm or more) and strengthenedglass articles each having a compressive stress layer with a very largethickness (30 μm or more) and a moderately high surface compressivestress (900 to 1100 MPa) could be obtained. In some Examples,strengthened glass articles each having a compressive stress layer witha very high surface compressive stress (1000 MPa or more) and a verylarge thickness (30 μm or more) could be obtained. In some otherExamples, strengthened glass articles each having a compressive stresslayer with an extremely high surface compressive stress (1200 MPa ormore or 1400 MPa or more) and strengthened glass articles each having acompressive stress layer with an extremely large thickness (40 μm ormore or 50 μm or more) and a moderately high surface compressive stress(900 to 1100 MPa) could be obtained.

Furthermore, in all Examples, strengthened glass articles each having acompressive stress layer with a surface compressive stress of 900 MPa ormore and a depth of 25 μm or more, and in addition, with a high crackinitiation load (3.9 kgf or more) could be obtained. In some of them,strengthened glass articles each having a compressive stress layer witha very high crack initiation load of 5 kgf or more, even 6 kgf or morein some cases could be obtained.

By contrast, in Comparative Examples 1 to 5, the surface compressivestresses were less than 900 MPa.

In Comparative Examples 6 to 9, no strengthened glass article having acompressive surface layer satisfying all of the requirements: a crackinitiation load of 3.9 kgf or more; a surface compressive stress of 900MPa or more; and a thickness of 2.5 μm or more, could be obtained.

TABLE 1 Examples 1 2 3 4 5 6 7 8 9 10 11 12 13 Composition SiO₂ 58.058.3 59.0 60.0 60.0 60.0 60.0 60.5 60.5 60.5 60.5 60.5 60.5 mol % ZrO₂ 00 0 0 0 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 00 0 0 0 0 0 0 Al₂O₃ 9.7 9.8 11.5 11.0 11.5 11.5 12.0 8.1 8.6 8.8 9.0 9.19.2 MgO 10.4 10.1 8.2 7.5 8.2 8.2 7.2 9.5 9.0 9.1 9.1 8.5 7.9 CaO 0.40.6 0.3 0 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.4 0.4 SrO 0.5 0 0 0 0 0 0 0 0 00 0 0 Na₂O 20.5 20.8 18.5 21.5 17.5 18.5 18.5 21.0 21.0 21.0 20.8 21.221.8 K₂O 0.5 0.4 2.5 0 2.5 1.5 2.0 0.5 0.5 0.2 0.2 0.3 0.2 SnO₂ 0 0 0 00 0 0 0 0 0 0 0 0 R₂O 21.0 21.2 21.0 21.5 20.0 20.0 20.5 21.5 21.5 21.221.0 21.5 22.0 TL/° C. 1071 1030 1037 967 1038 1022 1005 1020 1009 10081047 995 1026 T₂/° C. 1474 1483 1570 1566 1602 1597 1610 1486 1501 15081516 1514 1512 T₄/° C. 1074 1079 1140 1129 1159 1153 1165 1071 1082 10861092 1093 1092 Tg/° C. 577 579 587 589 596 600 596 554 558 564 573 563564 α/×10⁻⁷° C.⁻¹ 106 110 109 104 106 103 105 108 107 106 104 106 106d/q · cm⁻³ 2.51 2.50 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.492.49 E/GPa 73.8 73.3 74.1 72.3 74.4 74.2 74.1 71.8 71.9 72.1 72.5 72.171.7 Highest Rc/kgf 4.9 5.3 4.8 5.3 4.9 5.2 5.3 5.6 5.7 5.8 5.2 5.9 5.5crack CS/MPa 1122 1070 1155 1106 1174 1224 1187 952 960 1046 1000 988975 initiation DOL/μm 27.7 30.3 32.8 27.3 31.7 28.3 28.7 28.1 26.4 27.231.4 26.2 27.0 load I/E 420° C. 420° C. 400° C. 400° C. 400° C. 400° C.400° C. 400° C. 400° C. 400° C. 420° C. 400° C. 400° C. conditions 4 hr4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr CS CS/MPa1108 1070 1134 1091 1123 1181 1076 — — 1016 1000 — 1030 ≧1000 MPa DOL/μm33.9 30.3 42.0 33.4 38.3 36.0 41.3 — — 33.3 31.4 — 29.6 and I/E 420° C.420° C. 420° C. 400° C. 420° C. 420° C. 420° C. — — 400° C. 420° C. —380° C. DOL conditions 6 hr 4 hr 4 hr 6 hr 4 hr 4 hr 4 hr 6 hr 4 hr 8 hr≧30 μm DOL CS/MPa 1259 1323 1155 1106 1174 1224 1187 1031 1050 1046 1109988 1030 ≧25 μm DOL/μm 28.0 27.0 32.8 27.3 31.7 28.3 28.7 28.5 26.1 27.226.2 26.2 29.6 and I/E 400° C. 400° C. 400° C. 400° C. 400° C. 400° C.400° C. 380° C. 380° C. 400° C. 380° C. 400° C. 380° C. particularlyconditions 6 hr 6 hr 4 hr 4 hr 4 hr 4 hr 4 hr 8 hr 8 hr 4 hr 8 hr 4 hr 8hr high CS CS CS/MPa 1082 1020 1051 920 1033 1087 1036 922 936 943 925975 955 ≧900 MPa DOL/μm 39.2 48.6 59.4 52.0 56.5 50.9 58.4 39.8 37.445.0 44.3 37.0 38.2 and I/E 420° C. 420° C. 420° C. 420° C. 420° C. 420°C. 420° C. 400° C. 400° C. 420° C. 420° C. 400° C. 400° C. particularlyconditions 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8hr deep DOL

TABLE 2 Examples 14 15 16 17 18 19 20 21 22 23 24 25 26 Composition SiO₂60.5 61.0 61.0 61.0 61.0 61.0 61.0 61.8 61.8 61.8 62.0 62.4 60.0 mol %ZrO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 00 0 0 0 0 0 0 0 0 0 Al₂O₃ 9.2 8.5 8.5 8.5 8.8 11.5 11.5 8.3 8.3 8.4 10.511.3 9.3 MgO 7.9 8.6 9.1 8.9 8.3 7.2 7.2 8.5 8.2 7.9 7.5 7.2 9.3 CaO 0.40.4 0.4 0.4 0.4 0.3 0.3 0.4 0.7 0.4 0 0.3 0.4 SrO 0 0 0 0 0 0 0 0 0 0 00 0 Na₂O 21.3 20.9 20.8 21.0 21.0 17.5 18.5 20.8 20.8 21.3 20.0 18.821.0 K₂O 0.7 0.6 0.2 0.2 0.5 2.5 1.5 0.2 0.2 0.2 0 0 0 SnO₂ 0 0 0 0 0 00 0 0 0 0 0 0 R₂O 22.0 21.5 21.0 21.2 21.5 20.0 20.0 21.0 21.0 21.5 20.018.8 21.0 TL/° C. 1097 999 1007 922 1033 1011 996 984 965 974 873 9741013 T₂/° C. 1515 1510 1513 1498 1518 1625 1620 1525 1518 1523 1612 16501512 T₄/° C. 1072 1087 1088 1071 1094 1173 1168 1094 1093 1096 1152 11811091 Tg/° C. 557 555 563 566 558 591 596 556 557 553 594 612 574α/×10⁻⁷° C.⁻¹ 109 107 105 106 107 106 103 107 109 106 110 95 105 d/q ·cm⁻³ 2.49 2.49 2.49 2.49 2.49 2.48 2.48 2.48 2.49 2.49 2.48 2.47 2.49E/GPa 71.8 71.8 72.0 71.9 71.8 74.1 73.9 71.7 72.1 71.4 72.5 73.8 72.6Highest Rc/kgf 6.0 6.0 5.9 5.9 6.0 5.4 5.7 6.4 6.9 6.4 5.6 6.2 5.6 crackCS/MPa 960 984 1044 1024 955 1187 1173 944 993 928 1101 1208 1041initiation DOL/μm 28.5 28.4 25.5 26.0 27.4 32.9 29.6 25.4 25.0 26.5 26.430.3 30.0 load I/E 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400°C. 400° C. 400° C. 400° C. 400° C. 420° C. 420° C. conditions 4 hr 4 hr4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr CS CS/MPa 1027 —1014 990 — 1112 1111 — — — 1009 1208 1041 ≧1000 MPa DOL/μm 30.1 — 31.231.8 — 42.0 37.2 — — — 34.0 30.3 30.0 and I/E 380° C. — 400° C. 400° C.— 420° C. 420° C. — — — 420° C. 420° C. 420° C. DOL conditions 8 hr 6 hr6 hr 4 hr 4 hr 4 hr 4 hr 4 hr ≧30 μm DOL CS/MPa 1027 984 1044 1100 9551187 1173 944 993 980 1101 1251 1193 ≧25 μm DOL/μm 30.1 28.4 25.5 24.527.4 32.9 29.6 25.4 25.0 30.5 26.4 28.7 27.5 and I/E 380° C. 400° C.400° C. 380° C. 400° C. 400° C. 400° C. 400° C. 400° C. 380° C. 400° C.400° C. 400° C. particularly conditions 8 hr 4 hr 4 hr 8 hr 4 hr 4 hr 4hr 4 hr 4 hr 8 hr 4 hr 6 hr 6 hr high CS CS CS/MPa 947 905 908 922 9331072 1031 933 951 900 980 1186 1011 ≧900 MPa DOL/μm 40.3 35.2 44.6 39.638.7 59.4 53.7 35.9 35.3 37.5 48.1 42.9 42.5 and I/E 400° C. 420° C.420° C. 420° C. 400° C. 420° C. 420° C. 400° C. 400° C. 400° C. 420° C.420° C. 420° C. particularly conditions 8 hr 4 hr 8 hr 6 hr 8 hr 8 hr 8hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr deep DOL

TABLE 3 Examples 27 28 29 30 31 32 33 34 35 36 37 38 39 Composition SiO₂57.0 57.0 57.0 60.0 61.0 58.0 58.0 58.0 58.0 58.0 58.3 58.5 58.5 mol %ZrO₂ 0 0 0 0 0 0 0.5 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0.5 0 0 0 0 0 B₂O₃ 00 0 0 0 0 0 0 0 0 0 0 0 Al₂O₃ 12.0 11.0 10.0 7.8 8.5 9.7 9.7 9.7 9.711.5 9.8 9.7 10.0 MgO 7.2 9.6 12.0 10.8 8.9 10.9 10.4 10.4 10.3 9.6 9.910.1 9.8 CaO 0.3 0.4 0.5 0.4 0.4 0.4 0.4 0.4 1.0 0.4 0.8 0.4 0.4 SrO 0 00 0 0 0 0 0 0 0 0 0 0 Na₂O 23.5 22.0 19.0 20.2 21.0 20.5 20.5 20.5 20.517.5 20.8 21.0 21.0 K₂O 0 0 1.5 0.8 0.2 0.5 0.5 0.5 0.5 3.0 0.4 0.3 0.3SnO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 R₂O 23.5 22.0 20.5 21.0 21.2 21.0 21.021.0 21.0 20.5 21.2 21.3 21.3 TL/° C. 971 1026 1104 1058 1002 1074 10961064 1037 1076 1056 1006 1020 T₂/° C. 1501 1483 1474 1472 1511 1480 14811471 1467 1552 1479 1488 1497 T₄/° C. 1099 1083 1074 1061 1087 1073 10741074 1072 1130 1078 1079 1086 Tg/° C. 579 587 589 557 560 580 588 577571 594 578 578 580 α/×10⁻⁷° C.⁻¹ 108 107 111 108 106 105 106 106 101111 112 105 107 d/q · cm⁻³ 2.50 2.50 2.51 2.49 2.49 2.50 2.51 2.51 2.512.50 2.50 2.50 2.50 E/GPa 72.5 73.3 74.6 72.3 71.9 73.4 74.2 73.6 73.775.0 75.1 73.0 73.0 Highest Rc/kgf 5.4 4.8 3.9 5.1 5.9 4.5 4.2 4.9 5.94.2 5.7 5.0 5.0 crack CS/MPa 1220 1161 1193 886 963 1110 1186 1126 10931249 1074 1033 1089 initiation DOL/μm 28.5 30.9 28.7 35.1 35.6 30.1 29.129.6 28.1 31.7 30.1 30.9 31.8 load I/E 400° C. 420° C. 420° C. 420° C.400° C. 420° C. 420° C. 420° C. 420° C. 400° C. 420° C. 420° C. 420° C.conditions 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4hr CS CS/MPa 1064 1161 1179 1001 — 1110 1180 1114 1075 1062 1074 10331089 ≧1000 MPa DOL/μm 39.1 30.9 35.1 32.7 — 30.1 35.6 36.3 34.4 39.430.1 30.9 31.8 and I/E 420° C. 420° C. 420° C. 400° C. — 420° C. 420° C.420° C. 420° C. 420° C. 420° C. 420° C. 420° C. DOL conditions 4 hr 4 hr6 hr 8 hr 4 hr 6 hr 6 hr 6 hr 4 hr 4 hr 4 hr 4 hr ≧30 μm DOL CS/MPa 12201318 1356 1011 963 1280 1254 1243 1476 1249 1389 1243 1251 ≧25 μm DOL/μm28.5 28.3 28.1 28.3 35.6 26.9 25.7 27.7 24.7 31.7 26.3 27.8 28.3 and I/E400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C.400° C. 400° C. 400° C. 400° C. particularly conditions 4 hr 6 hr 6 hr 6hr 4 hr 6 hr 6 hr 6 hr 6 hr 4 hr 6 hr 6 hr 6 hr high CS CS CS/MPa 10341141 1173 904 922 1070 1164 1104 1053 1020 1034 1000 1049 ≧900 MPaDOL/μm 55.2 43.7 40.5 42.8 39.6 46.0 41.2 41.9 39.7 55.7 42.6 43.7 45.0and I/E 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C.420° C. 420° C. 420° C. 420° C. 420° C. particularly conditions 8 hr 8hr 8 hr 6 hr 6 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr deep DOL

TABLE 4 Examples 40 41 42 43 44 45 46 47 48 49 50 51 52 Composition SiO₂59.0 59.0 59.5 59.8 60.0 60.0 60.0 60.0 60.0 60.5 60.0 60.0 60.0 mol %ZrO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 00 0 0 0 0 0 0 0 0 0 Al₂O₃ 9.5 10.0 10.0 9.5 9.0 9.1 9.3 9.5 9.5 9.0 9.38.8 8.3 MgO 9.8 9.6 9.6 9.6 9.6 10.1 10.3 9.2 10.1 9.7 9.8 10.3 10.3 CaO0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.4 0.8 0.4 0.4 0.4 SrO 0 0 0 0 0 0 0 00 0 0 0 0 Na₂O 21.0 20.5 20.0 20.3 20.5 19.4 19.4 20.5 19.5 18.8 20.420.2 21.0 K₂O 0.3 0.5 0.5 0.5 0.5 1.0 0.6 0.5 0.5 1.2 0.1 0.3 0 SnO₂ 0 00 0 0 0 0 0 0 0 0 0 0 R₂O 21.3 21.0 20.5 20.8 21.0 20.4 20.0 21.0 20.020.0 20.5 20.5 21.0 TL/° C. 1013 1031 1032 921 915 1001 1042 975 1051927 1028 1042 1035 T₂/° C. 1494 1512 1529 1517 1506 1517 1536 1523 15301522 1517 1504 1483 T₄/° C. 1081 1095 1104 1095 1086 1094 1099 1098 11021100 1094 1083 1068 Tg/° C. 574 581 585 576 571 572 579 576 585 575 579574 565 α/×10⁻⁷° C.⁻¹ 107 106 105 102 108 106 104 106 104 111 104 105106 d/q · cm⁻³ 2.50 2.50 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.492.49 2.49 E/GPa 72.7 73.2 73.4 73.0 72.6 73.2 73.2 72.7 73.4 73.3 72.972.8 72.2 Highest Rc/kgf 5.2 5.1 5.2 5.3 5.4 5.1 5.1 5.3 5.2 6.0 5.4 5.25.4 crack CS/MPa 1014 1038 1062 1015 983 1012 1042 989 1070 1005 10751049 1008 initiation DOL/μm 32.7 32.7 30.7 32.6 33.4 33.2 30.3 33.5 29.231.3 28.4 28.4 28.4 load I/E 420° C. 420° C. 420° C. 420° C. 420° C.420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C.conditions 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4hr CS ≧1000 CS/MPa 1014 1038 1062 1015 1143 1012 1042 1140 1050 10051061 1035 1148 MPa and DOL/μm 32.7 32.7 30.7 32.6 33.5 33.2 30.3 30.236.1 31.3 34.8 34.8 30.0 DOL ≧30 I/E 420° C. 420° C. 420° C. 420° C.400° C. 420° C. 420° C. 400° C. 420° C. 420° C. 420° C. 420° C. 400° C.μm conditions 4 hr 4 hr 4 hr 4 hr 8 hr 4 hr 4 hr 6 hr 6 hr 4 hr 6 hr 6hr 8 hr DOL ≧25 CS/MPa 1213 1236 1246 1203 1161 1180 1233 1140 1243 12101186 1200 1169 μm and DOL/μm 28.3 29.1 28.4 28.7 29.1 29.5 26.9 30.226.9 26.5 26.3 27.0 25.8 particularly I/E 400° C. 400° C. 400° C. 400°C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400°C. high CS conditions 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr6 hr 6 hr 6 hr CS ≧900 CS/MPa 1079 1002 1010 958 921 962 1001 945 1029963 1045 1019 973 MPa and DOL/μm 46.2 45.0 44.5 45.8 48.6 46.0 42.6 48.242.0 43.9 40.2 40.1 40.2 particularly I/E 420° C. 420° C. 420° C. 420°C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420°C. deep DOL conditions 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr8 hr 8 hr 8 hr

TABLE 5 Examples 53 54 55 56 57 58 59 60 61 62 Composition SiO₂ 60.059.0 60.0 60.0 59.0 60.0 60.0 60.0 60.0 59.0 mol % ZrO₂ 0 0 0 0 0 0 0 00 0 TiO₂ 0 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 0 0 0 0 0 Al₂O₃ 8.3 10.3 8.87.8 8.3 7.8 8.8 8.3 8.3 10.3 MgO 10.8 9.3 9.8 10.3 10.3 10.8 10.3 10.810.3 9.3 CaO 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 SrO 0 0 0 0 0 0 0 00 0 Na₂O 20.1 21.0 21.0 20.5 21.5 20.0 20.1 19.5 20.2 20.8 K₂O 0.4 0 01.0 0.5 1.0 0.4 1.0 0.8 0.2 SnO₂ 0 0 0 0 0 0 0 0 0 0 R₂O 20.5 21.0 21.021.5 22.0 21.0 20.5 20.5 21.0 21.0 TL/° C. 1055 1017 1024 1048 1041 10611044 1064 1047 1020 T₂/° C. 1490 1518 1497 1469 1453 1473 1504 1493 14871520 T₄/° C. 1072 1099 1079 1059 1052 1062 1084 1075 1073 1100 Tg/° C.569 586 569 550 554 556 573 566 568 587 α/×10⁻⁷° C.⁻¹ 106 104 105 110110 109 105 107 107 104 d/q · cm⁻³ 2.49 2.49 2.49 2.50 2.50 2.50 2.492.49 2.49 2.49 E/GPa 72.7 73.2 72.4 72.0 72.0 72.3 72.8 73.0 72.6 72.9Highest crack Rc/kgf 5.1 5.3 5.5 5.2 5.1 5.0 5.2 5.0 5.2 5.3 initiationload CS/MPa 1028 1121 1024 1087 956 947 1007 1001 973 1112 DOL/μm 28.029.7 29.2 25.1 32.5 31.6 30.7 30.4 31.6 30.5 I/E 420° C. 420° C. 420° C.400° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. conditions 4 hr4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr 4 hr CS ≧1000 MPa CS/MPa 11601105 1007 1087 1107 1088 1000 1001 1011 1112 and DOL DOL/μm 30.1 34.336.3 25.1 33.8 33.7 35.2 30.4 33.7 30.5 ≧30 μm I/E 400° C. 420° C. 420°C. 400° C. 400° C. 400° C. 420° C. 420° C. 400° C. 420° C. conditions 8hr 6 hr 6 hr 6 hr 8 hr 8 hr 6 hr 4 hr 8 hr 4 hr DOL ≧25 μm CS/MPa 11841268 1181 1087 1125 1106 1116 1154 1029 1258 and DOL/μm 26.0 27.5 26.625.1 29.3 29.2 26.8 28.5 29.2 28.3 particularly I/E 400° C. 400° C. 400°C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. 400° C. high CSconditions 6 hr 6 hr 6 hr 4 hr 6 hr 6 hr 6 hr 6 hr 6 hr 6 hr CS ≧900 MPaCS/MPa 998 1101 994 910 904 924 980 966 938 1092 and DOL/μm 39.6 42.041.3 33.6 45.9 38.7 38.0 42.9 44.7 43.1 particularly I/E 420° C. 420° C.420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. deep DOLconditions 8 hr 8 hr 8 hr 4 hr 8 hr 6 hr 8 hr 8 hr 8 hr 8 hr

TABLE 6 Comparative Examples 1 2 3 4 5 6 7 8 9 Composition SiO₂ 75.070.0 70.0 65.0 60.0 55.0 55.0 66.0 63.0 mol % ZrO₂ 0 0 0 0 0 0 0 0.012.04 TiO₂ 0 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 0 0 0.6 1.9 Al₂O₃ 6.5 9.5 9.511.5 10.0 11.5 15.0 10.3 8.3 MgO 2.5 2.2 2.5 2.5 9.7 12.5 9.7 5.8 3.2CaO 0 0.3 0 0 0.3 0 0.3 0.6 2.3 SrO 0 0 0 0 0 0 0 0 0 Na₂O 16.0 15.318.0 21.0 13.2 21.0 20.0 14.2 15.6 K₂O 0 2.7 0 0 6.8 0 0 2.4 3.4 SnO₂ 00 0 0 0 0 0 0.14 0.13 R₂O 16.0 18.0 18.0 21.0 20.0 21.0 20.0 16.6 19.0TL/° C. 864 877 861 <833 1138 >1200 1053 <890 <878 T₂/° C. 1832 17941786 1701 1582 1471 1577 >1613 1561 T₄/° C. 1263 1264 1251 1213 11471072 1155 >1131 1122 Tg/° C. 549 557 577 578 583 607 647 606 569α/×10⁻⁷° C.⁻¹ 85 98 90 98 113 101 97 92 103 d/q · cm⁻³ 2.41 2.44 2.432.46 2.49 2.51 2.50 2.46 2.54 E/GPa 69.9 72.1 71.1 71.2 75.0 74.4 76.572.9 75.4 Highest Rc/kgf — — — — — 2.7 3.8 5.6 3.2 crack CS/MPa — — — —— 1266 1519 970 1022 initiation DOL/μm — — — — — 25.2 25.7 23.2 17.5load I/E — — — — — 420° C. 420° C. 380° C. 380° C. conditions 4 hr 4 hr4 hr 4 hr CS ≧1000 MPa CS/MPa — — — — — 1240 1505 Not 1011 and DOL/μm —— — — — 30.9 31.5 achieved 30.8 DOL ≧30 μm I/E — — — — — 420° C. 420° C.420° C. conditions 6 hr 6 hr 4 hr DOL ≧25 μm CS/MPa 585 737 775 845 8701340 1519 970 1011 and DOL/μm 27.5 36.6 28.9 33.1 45.5 27.2 25.7 23.230.8 particularly I/E 400° C. 400° C. 400° C. 400° C. 400° C. 400° C.420° C. 380° C. 420° C. high CS conditions 4 hr 4 hr 4 hr 4 hr 4 hr 8 hr4 hr 4 hr 4 hr CS ≧900 MPa CS/MPa 430 601 612 700 800 1230 1499 901 1011and DOL/μm 51.2 67.6 53.2 62.2 75.7 35.6 36.3 47.4 30.8 particularly I/E420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C. 420° C.deep DOL conditions 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 8 hr 4 hr

INDUSTRIAL APPLICABILITY

The present invention can provide a glass composition suitable forproduction by a float process and suitable for use, for example, in acover glass for a display.

1. A glass composition comprising, in mol %: 56 to 66% SiO₂; 6 to 12%Al₂O₃; 5 to 14% MgO; 0 to 1% CaO; 17 to 24% Na₂O; and 0 to 3% K₂O,wherein a total content of Li₂O, Na₂O, and K₂O is in a range of 18.5 to24%.
 2. The glass composition according to claim 1, wherein a content ofAl₂O₃ is 6 to 10 mol %.
 3. The glass composition according to claim 1,wherein a temperature T₄ at which the glass composition has a viscosityof 10⁴ dPa·s is 1100° C. or less.
 4. The glass composition according toclaim 1, wherein a temperature T₂ at which the glass composition has aviscosity of 10² dPa·s is 1550° C. or less.
 5. The glass compositionaccording to claim 1, wherein a difference obtained by subtracting aliquidus temperature TL from a temperature T₄ at which the glasscomposition has a viscosity of 10⁴ dPa·s is −10° C. or more.
 6. Theglass composition according to claim 1, consisting essentially of, inmol %: 57 to 64% SiO₂; 0 to 3% B₂O₃; 7 to 11% Al₂O₃; 7 to 12% MgO; 0 to1% CaO; 19 to 22% Na₂O; 0 to 1.5% K₂O; 0 to 1% TiO₂; 0 to 1% ZrO₂; 0.02%or less total iron oxide in terms of Fe₂O₃; 0.1 to 0.3% SO₃; and 0 to0.4% SnO₂, wherein a total content of Na₂O and K₂O is in a range of 19to 22%.
 7. The glass composition according to claim 6, wherein the glasscomposition is substantially free of B₂O₃.
 8. The glass compositionaccording to claim 6, wherein the glass composition is substantiallyfree of TiO₂.
 9. A glass composition for chemical strengthening, whereinthe glass composition for chemical strengthening is the glasscomposition according to claim 1 and is used in chemical strengtheningtreatment.
 10. A strengthened glass article comprising a compressivestress layer formed as a surface of the strengthened glass article bybringing the glass composition according to claim 1 into contact with amolten salt containing monovalent cations having an ionic radius largerthan that of sodium ions so as to allow ion exchange to take placebetween sodium ions contained in the glass composition and themonovalent cations.
 11. The strengthened glass article according toclaim 10, wherein the compressive stress layer has a surface compressivestress of 900 MPa or more and a depth of 25 μm or more, and in addition,has a crack initiation load of 3.9 kgf or more, the crack initiationload being defined as an indentation load at which cracks emanating froman indentation formed by a Vickers indenter occur with a probability of50%.
 12. The strengthened glass article according to claim 11, whereinthe compressive stress layer has a surface compressive stress of 1000MPa or more and a depth of 30 μm or more.
 13. A cover glass for adisplay, the cover glass comprising the strengthened glass articleaccording to claim 10.